This invention relates to Wavelength Division Multiplexed (WDM) optical transmission systems and more particularly to dispersion managed return-to-zero (RZ) WDM systems.
Dispersion managed WDM systems have been widely studied for use in large capacity digital transmission systems. Such dispersion managed systems typically involve a WDM optical system that includes a plurality of dispersion managed optical transmissions links. Each of these dispersion managed links includes one or more spans of dispersive fibers, the dispersion of which is compensated by a dispersion compensation module (DCM) connected between adjacent spans.
Dispersion management has proven to be an effective method for increasing the capacity of RZ WDM systems. Achieving improved system performance usually involves the creation of dispersion maps under a given set of predetermined system constraints, such as transmission rate, pulse width, peak power, and amplifier spacing.
A dispersion map shows, in a transmission link, the relationship between accumulated dispersion and distance, at a given wavelength. This relationship affects the quality of optical signals as they travel along the link, hence affecting the system performance.
In a WDM system, dispersion compensation is required not only at one wavelength, but rather at a wide range of wavelengths. To achieve this, the relative dispersion slope (RDS) of the transmission fibers and DCMs must be properly matched. When the RDSs of the fibers and DCMs are equal, dispersion compensation can be achieved simultaneously for all wavelengths. At the end of the transmission link there is no residual slope on the accumulated dispersion with respect to wavelength. However, if the RDSs are mismatched, a residual slope develops, such that the shape of the dispersion map varies with wavelength. Residual slope also causes variation in the dispersion maps. This variation becomes an important factor in WDM system optimization.
Major system impairments are due to non-linear effects such as self-phase modulation (SPM) and inter-symbol interference (ISI), which are chief concerns in single-channel systems, as well as cross-phase modulation (XPM) and four wave mixing (FWM), which additionally affect multi-channel systems. In known WDM systems, dispersion managed links that minimize SPM tend to maximize XPM, and vice versa. Without addressing the trade-off between SPM, ISI, and XPM effects, the overall performance of the WDM system can be penalized.
It is an object of the present invention to provide an improved dispersion managed link for RZ WDM optical systems.
The selection of the fibers and DCMs for the improved link is directed to minimizing the maximum combined effect, occurring at a WDM wavelength, of SPM, ISI and XPM. In this way the invention addresses the trade-off between SPM, ISI, and XPM effects.
According to an aspect of the present invention there is provided a dispersion managed link for transmitting wavelength division multiplexed optical signals having a range of predetermined wavelengths and a center wavelength. The link includes a plurality of spans, each span having a length of optically dispersive fiber and a dispersion compensating module connected to the fiber. The link also includes a plurality of optical amplifiers, each optical amplifier serially connecting adjacent spans. Each span has a maximum dispersion and a minimum dispersion at the center wavelength. The maximum and minimum dispersions have respective distributions of maximum and minimum dispersion. The distributions have one or more portions where the maximum dispersions are changing in accordance with the minimum dispersions for changes in location along the link, whereby a maximum combined effect of self-phase modulation, inter-symbol interference, and cross-phase modulation within the range is minimized.
Embodiments of this aspect of the invention have less adverse effects on the Q-value of signals transmitted over them than do traditional dispersion managed links. This leads to an improvement in overall performance of WDM optical systems that employ these improved links.
According to another aspect of the present invention there is provided a method of providing a dispersion managed link for a wavelength division multiplexed optical transmission system having a plurality of predetermined wavelengths one of which is a center wavelength. The link includes a plurality of serially connected spans. Each span has a length of optically dispersive fiber connected to a dispersion compensating module and is connected to an adjacent span by an optical amplifier. The method comprises the steps of:
a. selecting one of a ramp type, angular type or arcuate type dispersion map for representing dispersion on the link;
b. generating a set of dispersion maps, of the type selected, at the center wavelength;
c. simulating, for each map in the set, each wavelength to obtain a performance indicator of that wavelength;
d. determining, for each map in the set, which one of the performance indicators is the minimum performance indicator;
e. selecting the map with the greatest minimum performance indicator; and
f. selecting dispersive fibers and dispersion compensating modules for each span in accordance with the selected map.
Embodiments of this aspect of the invention provide dispersion managed links that have less adverse effects on the Q-value of signals transmitted over them than do traditional dispersion managed links. By selecting the map with the greatest minimum performance indicator, the maximum combined effect, occurring at one of the predetermined wavelengths, of self-phase modulation, inter-symbol interference, and cross-phase modulation is minimized. This leads to an improvement in overall performance of WDM optical systems that employ these improved links.